For fulfilling the requests for expanding or colorizing a display and improving response time as well as display qualities, such as contrast and color development, technologies have been developed from twisted nematic (TN) to super twisted nematic (STN) and further to a TFT display element having thin filmed transistors (TFTs) on respective pixels. In recent years, improvements in drive mechanisms of the TFT display elements have been progressed. To extend the viewing angle more, for instance, the in-plain switching (IPS) mode and the vertical aliment (hereinafter, abbreviated as VA) mode have been developed, and furthermore the optically compensated bend (OCB) mode having a response time capable of responding to a moving image has been developed.
From a standpoint of applications, presently, liquid crystal displays have been applied extensively from PC monitors to TV. Therefore, wider-screen displays have been demanded, while it becomes very difficult to keep constantly an image quality. In addition, according to applications on TV compared with the applications on monitors, it has been requested to prevent an image from being troubled even after driving for a long time (i.e., retaining high reliability) as the user uses it for a prolonged time.
The liquid crystal alignment film plays two roles: one for aligning a liquid crystal composition in a given direction in a liquid crystal display element; and the other for tilting the liquid crystal composition against the plane of the substrate (providing a pre-tilt angle). The slope of the liquid crystal composition against the plane of the substrate is referred to as “pre-tilt angle”. In the present description, such a name will be used below. The liquid crystal alignment film is prepared using a polyimide thin film as a primary material, which has a high glass transition point (Tg) and excellent in chemical resistance and heat resistance to minimize temporal, chemical, and thermal deteriorations of the molecular alignment. The liquid crystal alignment film is provided with a polyimide thin film, which is in general obtained by applying a polyamic acid or soluble polyimide solution (hereinafter, abbreviated as “varnish”) on a glass substrate with electrodes by the spinner method, printing method, or the like and heating the substrate to dehydrate polyamic acid and close the ring thereof or vaporize a solvent in the soluble polyimide solution. Further, the thin film is rubbed by cloth folded on a roller to provide the liquid crystal alignment film with uniform alignment characteristics to a liquid crystal composition.
However, the above-mentioned alignment treatment with rubbing causes several problems as described below when a liquid crystal display element is manufactured. Characteristics of a liquid crystal display element, such as a voltage holding ratio (VHR), may be decreased due to static electricity caused by rubbing operation, contaminants adsorbed on the surface of an alignment film by the static electricity, or impurities eluted from cloth by rubbing and thus the reliabilities of the display may be lowered. In addition, a larger-sized display may generate an area where the rubbing cloth does not contact with the liquid crystal alignment film in the case of deforming a substrate while rubbing thereof. In contrast, a high quality display having small-sized pixels leads to a large difference of elevation between the electrodes, so it may result in display irregularity due to a difficulty in uniform alignment treatment.
Methods for noncontact alignment treatment using light or charged particles (so-called rubbingless) have been demanded. However, these methods may result in insufficient alignment properties of liquid crystals in practical use. In addition, the characteristics, such as VHR, may deteriorate because of attributing to materials or the treatment.
Regarding the noncontact alignment treatment, for example, a photo-alignment film made of a polymer having a double or triple bond in main chain has been known in the art (for example, see Patent Document 1). However, even though the heat resistance of the photo-alignment film has been studied, the document does not describe the electric characteristics of a prepared alignment film. Thus, needs more consideration for other characteristics, such as VHR, for the dependability of the liquid crystal display element still remain.
Further, for the noncontact alignment treatment, it is known that a film having extremely excellent alignment properties can be obtained by irradiating light on a polyamic acid having azobenzene as a main chain of the polyamic acid before imidization and then imidizing the polyamic acid (for example, see Non-Patent Document 1). However, the document does not describe electric properties of an alignment film prepared.    [Patent Document 1] JP 11-15001 A    [Non Patent Document 1] Mol. Cryst. Liq. Cryst. Vol. 412, 293 (2004)